In the present document, the term “dose” if not further specified may relate to either “energy dose” or “equivalent dose”. The energy dose is equal to the energy deposited per unit mass of a medium and is measured in units of J/kg, which is denoted as Gray (Gy). However, the biological effect of radiation can not be estimated by the energy dose only. Heavy particles are in general more harmful than an identical dose of X-ray, gamma or beta radiation. With regard to the human body, this is accounted for by a quality factor or radiation weighting factor denoted as wr, which compares the relative biological effects of various types of radiation. Accordingly, an equivalent dose is defined by the product of the energy dose and the weighting factor wr, and it is measured in units called Sievert (Sv). The quality factors wr range from 1 for X-ray, gamma and beta radiation, 10 for alpha particles up to 20 for heavy nuclei. In dosimetry, it is therefore usually not sufficient to simply measure the energy dose but it is also necessary to somehow distinguish the contributions of different types or categories of radiation of the radiation environment.
The oldest way of measuring radiation dose is based on blackening of silver grains in a sensitive emulsion, which requires development of the film and subsequent measurement of the blackening. An advantage of the emulsion is the visible difference in ionization and track characteristics for different types of radiation, such as alpha particles, heavy ions, cosmic muons or electrons. This blackening of silver grains in a sensitive emulsion is an example of a passive detector, which is exposed to radiation and afterwards analyzed to determine the absorbed dose. Other well known examples for passive detectors are thermo luminescent detectors, alanine detectors, gel and radio-photo luminescent detectors, and also track-sensitive plastic detectors. Passive detectors do not allow time resolved measurements. Also, passive detectors accumulate background before and after the intended exposure itself, and some of them are subject to fading.
On the other hand, there are known active detectors which have the ability of time resolved measurement which is in many applications a desired feature. In an active detector, generally some electrical current signals resulting from an ionization process caused by ionizing radiation in a suitable detector material such as a gas or semiconductors are analyzed.
A typical active detector using gas as the sensitive material is the ionization chamber, which is frequently used nowadays, for example in the Geiger-Mueller-Counter. An example for a semiconductor sensitive material is a Si-diode, which is used in some commercially available dosimeters. Both examples of active detectors allow to a certain extent the evaluation of the linear energy transfer (LET) spectra.
As mentioned above, for an assessment of the equivalent dose, one has to distinguish radiation according to both, type and energy. As long as the composition of the ionizing radiation is not determined, it remains a difficult issue of calibration to convert a measured electrical signal into an equivalent dose or effective biological damage factor, such that simple dosimeters have a problem of giving an imprecise estimate of the equivalent dose. On the other hand, known spectroscopy techniques for distinguishing radiation according to type and energy require different varieties of detectors and lead to complicated, bulky and expensive apparatuses. When it comes to determining the biological damage factor using traditional measurement equipment, this can currently only be achieved to some degree by use of mechanical filters at the entrance of the measurement apparatus, and at the costs of added complexity and loss of sensitivity.
Recently, new active electronic methods have been developed that use the change of transistor characteristics under ionizing irradiation which have some proportionality to the energy dose and that allow remote measurements in various environments, cf. for example “Handbook of Radiation Effects”, 2nd edition, Andrew Holmes-Siedle and Len Adams, Oxford University Press, ISBN 0-9-850733-X. However, these methods do not resort to the quantum nature of radiation and do not allow to determine the components of the radiation.
An even more recent development has been the measurement of alpha particle radiation emitters such as gaseous radon by the use of a highly segmented semiconductor imager device, as shown in “First measurement of 222Rn activity with a CMOS active pixel sensor”, A. Nachab et al., Nucl. Instr. Meth. B 225 (2004), pages 418-422. This work has some similarity with the method of the invention. However, until now a charge coupled device (CCD) or a monolithic CMOS active imager accumulates signal charge in a pixel over a relatively long period and the integration time window is not the same for all pixels in the matrix. These devices do not provide signal processing in the pixels and background charge can not be eliminated. The use for radiation measurements is very limited because only some types of radiation can be recognized. A monolithic active pixel dosimeter is known from US 2006/0043313.
Until now, detectors used for dosimetry or radiation protection purposes are not able to determine all the necessary information needed for measuring an equivalent dose. In particular, known detectors and measuring methods do not allow to obtain the composition of the radiation field and LET values with the desired precision.
The object of the invention is to provide a method and an apparatus for measuring the dose, the dose rate and/or the composition of radiation with higher accuracy and with a more economic effort in equipment and time.